The plant-specific DDR factor SOG1 increases chromatin mobility in response to DNA damage.

Homologous recombination (HR) is a conservative DNA repair pathway in which intact homologous sequences are used as a template for repair. How the homology search happens in the crowded space of the cell nucleus is, however, still poorly understood. Here, we measure chromosome and double-strand break (DSB) site mobility in Arabidopsis thaliana, using lacO/LacI lines and two GFP-tagged HR reporters. We observe an increase in chromatin mobility upon the induction of DNA damage, specifically at the S/G2 phases of the cell cycle. This increase in mobility is lost in the sog1-1 mutant, a central transcription factor of the DNA damage response in plants. Also, DSB sites show particularly high mobility levels and their enhanced mobility requires the HR factor RAD54. Our data suggest that repair mechanisms promote chromatin mobility upon DNA damage, implying a role of this process in the early steps of the DNA damage response.

Chromosome-specific NOR inactivation explains selective rRNA gene silencing and dosage control in Arabidopsis.

In eukaryotes, scores of excess ribosomal RNA (rRNA) genes are silenced by repressive chromatin modifications. Given the near sequence identity of rRNA genes within a species, it is unclear how specific rRNA genes are reproducibly chosen for silencing. Using Arabidopsis thaliana ecotype (strain) Col-0, a systematic search identified sequence polymorphisms that differ between active and developmentally silenced rRNA gene subtypes. Recombinant inbred mapping populations derived from three different ecotype crosses were then used to map the chromosomal locations of silenced and active RNA gene subtypes. Importantly, silenced and active rRNA gene subtypes are not intermingled. All silenced rRNA gene subtypes mapped to the nucleolus organizer region (NOR) on chromosome 2 (NOR2). All active rRNA gene subtypes mapped to NOR4. Using an engineered A. thaliana line in which a portion of Col-0 chromosome 4 was replaced by sequences of another ecotype, we show that a major rRNA gene subtype silenced at NOR2 is active when introgressed into the genome at NOR4. Collectively, these results reveal that selective rRNA gene silencing is not regulated gene by gene based on mechanisms dependent on subtle gene sequence variation. Instead, we propose that a subchromosomal silencing mechanism operates on a multimegabase scale to inactivate NOR2.

Large tandem duplications affect gene expression, 3D organization, and plant-pathogen response.

Rapid plant genome evolution is crucial to adapt to environmental changes. Chromosomal rearrangements and gene copy number variation (CNV) are two important tools for genome evolution and sources for the creation of new genes. However, their emergence takes many generations. In this study, we show that in Arabidopsis thaliana, a significant loss of ribosomal RNA (rRNA) genes with a past history of a mutation for the chromatin assembly factor 1 (CAF1) complex causes rapid changes in the genome structure. Using long-read sequencing and microscopic approaches, we have identified up to 15 independent large tandem duplications in direct orientation (TDDOs) ranging from 60 kb to 1.44 Mb. Our data suggest that these TDDOs appeared within a few generations, leading to the duplication of hundreds of genes. By subsequently focusing on a line only containing 20% of rRNA gene copies (20rDNA line), we investigated the impact of TDDOs on 3D genome organization, gene expression, and cytosine methylation. We found that duplicated genes often accumulate more transcripts. Among them, several are involved in plant-pathogen response, which could explain why the 20rDNA line is hyper-resistant to both bacterial and nematode infections. Finally, we show that the TDDOs create gene fusions and/or truncations and discuss their potential implications for the evolution of plant genomes.

Glutathione-mediated thermomorphogenesis and heat stress responses in Arabidopsis thaliana.

In the context of climate change, the global rise of temperature and intense heat waves affect plant development and productivity. Among the molecular perturbations that high temperature induces in living cells is the accumulation of reactive oxygen species (ROS), which perturbs the cellular redox state. In plants, the dynamics of the cellular and subcellular redox state have been poorly investigated under high temperature. Glutathione plays a major role in maintaining the cellular redox state. We investigated its contribution in adaptation of Arabidopsis thaliana to contrasting high temperature regimes: high ambient temperature inducing thermomorphogenesis and heat stress affecting plant viability. Using the genetically encoded redox marker roGFP2, we show that high temperature regimes lead to cytoplasmic and nuclear oxidation and impact the glutathione pool. This pool is restored within a few hours, which probably contributes to plant adaptation to high temperatures. Moreover, low glutathione mutants fail to adapt to heat stress and to induce thermomorphogenesis, suggesting that glutathione is involved in both heat adaptation mechanisms. We also evaluate the transcriptomic signature in the two high temperature regimes and identified gene expression deviations in low glutathione mutants, which might contribute to their sensitivity to high temperature. Thus, we define glutathione as a major player in the adaptation of Arabidopsis to contrasting high temperature regimes.

NICOTIANAMINE SYNTHASE activity affects nucleolar iron accumulation and impacts rDNA silencing and RNA methylation in Arabidopsis.

In plant cells, a large pool of iron (Fe) is contained in the nucleolus, as well as in chloroplasts and mitochondria. A central determinant for intracellular distribution of Fe is nicotianamine (NA) generated by NICOTIANAMINE SYNTHASE (NAS). Here, we used Arabidopsis thaliana plants with disrupted NAS genes to study the accumulation of nucleolar iron and understand its role in nucleolar functions and more specifically in rRNA gene expression. We found that nas124 triple mutant plants, which contained lower quantities of the iron ligand NA, also contained less iron in the nucleolus. This was concurrent with the expression of normally silenced rRNA genes from nucleolar organizer regions 2 (NOR2). Notably, in nas234 triple mutant plants, which also contained lower quantities of NA, nucleolar iron and rDNA expression were not affected. In contrast, in both nas124 and nas234, specific RNA modifications were differentially regulated in a genotype dependent manner. Taken together, our results highlight the impact of specific NAS activities in RNA gene expression. We discuss the interplay between NA and nucleolar iron with rDNA functional organization and RNA methylation.

A two-step process for epigenetic inheritance in Arabidopsis.

In Arabidopsis, multisubunit RNA polymerases IV and V orchestrate RNA-directed DNA methylation (RdDM) and transcriptional silencing, but what identifies the loci to be silenced is unclear. We show that heritable silent locus identity at a specific subset of RdDM targets requires HISTONE DEACETYLASE 6 (HDA6) acting upstream of Pol IV recruitment and siRNA biogenesis. At these loci, epigenetic memory conferring silent locus identity is erased in hda6 mutants such that restoration of HDA6 activity cannot restore siRNA biogenesis or silencing. Silent locus identity is similarly lost in mutants for the cytosine maintenance methyltransferase, MET1. By contrast, pol IV or pol V mutants disrupt silencing without erasing silent locus identity, allowing restoration of Pol IV or Pol V function to restore silencing. Collectively, these observations indicate that silent locus specification and silencing are separable steps that together account for epigenetic inheritance of the silenced state.

An Effector from the Cyst Nematode Heterodera schachtii Derepresses Host rRNA Genes by Altering Histone Acetylation.

Cyst nematodes are plant-pathogenic animals that secrete effector proteins into plant root cells to alter host gene expression and reprogram these cells to form specialized feeding sites, known as syncytia. The molecular mechanisms of these effectors are mostly unknown. We determined that the sugar beet cyst nematode (Heterodera schachtii) 32E03 effector protein strongly inhibits the activities of Arabidopsis thaliana histone deacetylases including the HDT1 enzyme, which has a known function in the regulation of rRNA gene expression through chromatin modifications. We determined that plants expressing the 32E03 coding sequence exhibited increased acetylation of histone H3 along the rDNA chromatin. At low 32E03 expression levels, these chromatin changes triggered the derepression of a subset of rRNA genes, which were conducive to H. schachtii parasitism. By contrast, high levels of 32E03 caused profound bidirectional transcription along the rDNA, which triggered rDNA-specific small RNA production leading to RNA-directed DNA methylation and silencing of rDNA, which inhibited nematode development. Our data show that the 32E03 effector alters plant rRNA gene expression by modulating rDNA chromatin in a dose-dependent manner. Thus, the 32E03 effector epigenetically regulates plant gene expression to promote cyst nematode parasitism.

Subnuclear partitioning of rRNA genes between the nucleolus and nucleoplasm reflects alternative epiallelic states.

Eukaryotes can have thousands of 45S ribosomal RNA (rRNA) genes, many of which are silenced during development. Using fluorescence-activated sorting techniques, we show that active rRNA genes in Arabidopsis thaliana are present within sorted nucleoli, whereas silenced rRNA genes are excluded. DNA methyltransferase (met1), histone deacetylase (hda6), or chromatin assembly (caf1) mutants that disrupt silencing abrogate this nucleoplasmic-nucleolar partitioning. Bisulfite sequencing data indicate that active nucleolar rRNA genes are nearly completely demethylated at promoter CGs, whereas silenced genes are nearly fully methylated. Collectively, the data reveal that rRNA genes occupy distinct but changeable nuclear territories according to their epigenetic state.

Tidying-up the plant nuclear space: domains, functions, and dynamics.

Understanding how the packaging of chromatin in the nucleus is regulated and organized to guide complex cellular and developmental programmes, as well as responses to environmental cues is a major question in biology. Technological advances have allowed remarkable progress within this field over the last years. However, we still know very little about how the 3D genome organization within the cell nucleus contributes to the regulation of gene expression. The nuclear space is compartmentalized in several domains such as the nucleolus, chromocentres, telomeres, protein bodies, and the nuclear periphery without the presence of a membrane around these domains. The role of these domains and their possible impact on nuclear activities is currently under intense investigation. In this review, we discuss new data from research in plants that clarify functional links between the organization of different nuclear domains and plant genome function with an emphasis on the potential of this organization for gene regulation.

Three-dimensional nuclear organization in Arabidopsis thaliana.

In recent years, the study of plant three-dimensional nuclear architecture received increasing attention. Enabled by technological advances, our knowledge on nuclear architecture has greatly increased and we can now access large data sets describing its manifold aspects. The principles of nuclear organization in plants do not significantly differ from those in animals. Plant nuclear organization comprises various scales, ranging from gene loops to topologically associating domains to nuclear compartmentalization. However, whether plant three-dimensional chromosomal features also exert similar functions as in animals is less clear. This review discusses recent advances in the fields of three-dimensional chromosome folding and nuclear compartmentalization and describes a novel silencing mechanism, which is closely linked to nuclear architecture.

Nucleolus-associated chromatin domains are maintained under heat stress, despite nucleolar reorganization in Arabidopsis thaliana.

Several layers of mechanisms participate in plant adaptation to heat-stress. For example, the plant metabolism switches from cell growth mode to stress adaptation mode. Ribosome biogenesis is one of the most energy costly pathways. That biogenesis process occurs in the nucleolus, the largest nuclear compartment, whose structure is highly dependent on this pathway. We used a nucleolar marker to track the structure of the nucleolus, and revealed a change in its sub-nucleolar distribution under heat stress. In addition, the nucleolus is implicated in other cellular processes, such as genome organization within the nucleus. However, our analyses of nucleolus-associated chromatin domains under heat stress did not reveal significant differences compared to the control plants, suggesting a lack of connection between two of the main functions of the nucleolus: ribosome biogenesis and nuclear organization.

Hybrid incompatibility caused by an epiallele.

Hybrid incompatibility resulting from deleterious gene combinations is thought to be an important step toward reproductive isolation and speciation. Here, we demonstrate involvement of a silent epiallele in hybrid incompatibility. In Arabidopsis thaliana accession Cvi-0, one of the two copies of a duplicated histidine biosynthesis gene, HISN6A, is mutated, making HISN6B essential. In contrast, in accession Col-0, HISN6A is essential because HISN6B is not expressed. Owing to these differences, Cvi-0 × Col-0 hybrid progeny that are homozygous for both Cvi-0 HISN6A and Col-0 HISN6B do not survive. We show that HISN6B of Col-0 is not a defective pseudogene, but a stably silenced epiallele. Mutating HISTONE DEACETYLASE 6 (HDA6), or the cytosine methyltransferase genes MET1 or CMT3, erases HISN6B's silent locus identity, reanimating the gene to circumvent hisn6a lethality and hybrid incompatibility. These results show that HISN6-dependent hybrid lethality is a revertible epigenetic phenomenon and provide additional evidence that epigenetic variation has the potential to limit gene flow between diverging populations of a species.

Histone methyltransferases regulating rRNA gene dose and dosage control in Arabidopsis.

Eukaryotes have hundreds of nearly identical 45S ribosomal RNA (rRNA) genes, each encoding the 18S, 5.8S, and 25S catalytic rRNAs. Because cellular demands for ribosomes and protein synthesis vary during development, the number of active rRNA genes is subject to dosage control. In genetic hybrids, one manifestation of dosage control is nucleolar dominance, an epigenetic phenomenon in which the rRNA genes of one progenitor are repressed. For instance, in Arabidopsis suecica, the allotetraploid hybrid of Arabidopsis thaliana and Arabidopsis arenosa, the A. thaliana-derived rRNA genes are selectively silenced. An analogous phenomenon occurs in nonhybrid A. thaliana, in which specific classes of rRNA gene variants are inactivated. An RNA-mediated knockdown screen identified SUVR4 {SUPPRESSOR OF VARIEGATION 3-9 [SU(VAR)3-9]-RELATED 4} as a histone H3 Lys 9 (H3K9) methyltransferase required for nucleolar dominance in A. suecica. H3K9 methyltransferases are also required for variant-specific silencing in A. thaliana, but SUVH5 [SU(VAR)3-9 HOMOLOG 5] and SUVH6, rather than SUVR4, are the key activities in this genomic context. Mutations disrupting the H3K27 methyltransferases ATXR5 or ATXR6 affect which rRNA gene variants are expressed or silenced, and in atxr5 atxr6 double mutants, dominance relationships among variants are reversed relative to wild type. Interestingly, these changes in gene expression are accompanied by changes in the relative abundance of the rRNA gene variants at the DNA level, including overreplication of the normally silenced class and decreased abundance of the normally dominant class. Collectively, our results indicate that histone methylation can affect both the doses of different variants and their differential silencing through the choice mechanisms that achieve dosage control.

Selective nucleolus organizer inactivation in Arabidopsis is a chromosome position-effect phenomenon.

Nucleolus organizer regions (NORs) are chromosomal loci where hundreds of rRNA genes are clustered. Despite being nearly identical in sequence, specific rRNA genes are selected for silencing during development via choice mechanism(s) that remain unclear. In Arabidopsis thaliana, rRNA gene subtypes that are silenced during development were recently mapped to the NOR on chromosome 2, NOR2, whereas active rRNA genes map to NOR4, on chromosome 4. In a mutant line deficient for ATXR5 or ATXR6-dependent histone H3 lysine 27 (H3K27) monomethylation, we show that millions of base pairs of chromosome 4, including the telomere, TEL4N, and much of NOR4, have been converted to the corresponding sequences of chromosome 2. This genomic change places rRNA genes of NOR2, which are normally silenced, at the position on chromosome 4 where active rRNA genes are normally located. At their new location, NOR2-derived rRNA genes escape silencing, independent of the atxr mutations, indicating that selective rRNA gene silencing is chromosome 2-specific. The chromosome 2 position effect is not explained by the NOR2-associated telomere, TEL2N, which remains linked to the translocated NOR, implicating centromere-proximal sequences in silencing.

Mechanisms of HDA6-mediated rRNA gene silencing: suppression of intergenic Pol II transcription and differential effects on maintenance versus siRNA-directed cytosine methylation.

The Arabidopsis histone deacetylase HDA6 is required to silence transgenes, transposons, and ribosomal RNA (rRNA) genes subjected to nucleolar dominance in genetic hybrids. In nonhybrid Arabidopsis thaliana, we show that a class of 45S rRNA gene variants that is normally inactivated during development fails to be silenced in hda6 mutants. In these mutants, symmetric cytosine methylation at CG and CHG motifs is reduced, and spurious RNA polymerase II (Pol II) transcription occurs throughout the intergenic spacers. The resulting sense and antisense spacer transcripts facilitate a massive overproduction of siRNAs that, in turn, direct de novo cytosine methylation of corresponding gene sequences. However, the resulting de novo DNA methylation fails to suppress Pol I or Pol II transcription in the absence of HDA6 activity; instead, euchromatic histone modifications typical of active genes accumulate. Collectively, the data reveal a futile cycle of unregulated transcription, siRNA production, and siRNA-directed DNA methylation in the absence of HDA6-mediated histone deacetylation. We propose that spurious Pol II transcription throughout the intergenic spacers in hda6 mutants, combined with losses of histone deacetylase activity and/or maintenance DNA methylation, eliminates repressive chromatin modifications needed for developmental rRNA gene dosage control.

A duplicated NUCLEOLIN gene with antagonistic activity is required for chromatin organization of silent 45S rDNA in Arabidopsis.

In plants as well as in animals, hundreds to thousands of 45S rRNA gene copies localize in Nucleolus Organizer Regions (NORs), and the activation or repression of specific sets of rDNA depends on epigenetic mechanisms. Previously, we reported that the Arabidopsis thaliana nucleolin protein NUC1, an abundant and evolutionarily conserved nucleolar protein in eukaryotic organisms, is required for maintaining DNA methylation levels and for controlling the expression of specific rDNA variants in Arabidopsis. Interestingly, in contrast with animal or yeast cells, plants contain a second nucleolin gene. Here, we report that Arabidopsis NUC1 and NUC2 nucleolin genes are both required for plant growth and survival and that NUC2 disruption represses flowering. However, these genes seem to be functionally antagonistic. In contrast with NUC1, disruption of NUC2 induces CG hypermethylation of rDNA and NOR association with the nucleolus. Moreover, NUC2 loss of function triggers major changes in rDNA spatial organization, expression, and transgenerational stability. Our analyses indicate that silencing of specific rRNA genes is mostly determined by the active or repressed state of the NORs and that nucleolin proteins play a key role in the developmental control of this process.

Subunit compositions of Arabidopsis RNA polymerases I and III reveal Pol I- and Pol III-specific forms of the AC40 subunit and alternative forms of the C53 subunit.

Using affinity purification and mass spectrometry, we identified the subunits of Arabidopsis thaliana multisubunit RNA polymerases I and III (abbreviated as Pol I and Pol III), the first analysis of their physical compositions in plants. In all eukaryotes examined to date, AC40 and AC19 subunits are common to Pol I (a.k.a. Pol A) and Pol III (a.k.a. Pol C) and are encoded by single genes. Surprisingly, A. thaliana and related species express two distinct AC40 paralogs, one of which assembles into Pol I and the other of which assembles into Pol III. Changes at eight amino acid positions correlate with the functional divergence of Pol I- and Pol III-specific AC40 paralogs. Two genes encode homologs of the yeast C53 subunit and either protein can assemble into Pol III. By contrast, only one of two potential C17 variants, and one of two potential C31 variants were detected in Pol III. We introduce a new nomenclature system for plant Pol I and Pol III subunits in which the 12 subunits that are structurally and functionally homologous among Pols I through V are assigned equivalent numbers.

The overlooked manipulation of nucleolar functions by plant pathogen effectors.

Pathogens need to manipulate plant functions to facilitate the invasion of their hosts. They do this by secreting a cocktail of molecules called effectors. Studies of these molecules have mostly focused on the mechanisms underlying their recognition and the subsequent transcriptional reprogramming of cells, particularly in the case of R gene-dependent resistance. However, the roles of these effectors are complex, as they target all cell compartments and their plant targets remain largely uncharacterized. An understanding of the mechanisms involved would be a considerable asset for plant breeding. The nucleolus is the site of many key cellular functions, such as ribosome biogenesis, cellular stress regulation and many other functions that could be targets for pathogenicity. However, little attention has been paid to effectors targeting nucleolar functions. In this review, we aim to fill this gap by providing recent findings on pathogen effectors that target and manipulate nucleolar functions and dynamics to promote infection. In particular, we look at how some effectors hijack ribosome biogenesis, the modulation of transcription or alternative splicing, all key functions occurring at least partially in the nucleolus. By shedding light on the role of the plant nucleolus in pathogen interactions, this review highlights the importance of understanding nucleolar biology in the context of plant immunity and the mechanisms manipulated by plant pathogens.

Nucleolin is required for DNA methylation state and the expression of rRNA gene variants in Arabidopsis thaliana.

In eukaryotes, 45S rRNA genes are arranged in tandem arrays in copy numbers ranging from several hundred to several thousand in plants. Although it is clear that not all copies are transcribed under normal growth conditions, the molecular basis controlling the expression of specific sets of rRNA genes remains unclear. Here, we report four major rRNA gene variants in Arabidopsis thaliana. Interestingly, while transcription of one of these rRNA variants is induced, the others are either repressed or remain unaltered in A. thaliana plants with a disrupted nucleolin-like protein gene (Atnuc-L1). Remarkably, the most highly represented rRNA gene variant, which is inactive in WT plants, is reactivated in Atnuc-L1 mutants. We show that accumulated pre-rRNAs originate from RNA Pol I transcription and are processed accurately. Moreover, we show that disruption of the AtNUC-L1 gene induces loss of symmetrical DNA methylation without affecting histone epigenetic marks at rRNA genes. Collectively, these data reveal a novel mechanism for rRNA gene transcriptional regulation in which the nucleolin protein plays a major role in controlling active and repressed rRNA gene variants in Arabidopsis.